1. Technical Field
The present invention relates to a display using electroluminescence (hereinafter abbreviated to EL) elements, and in particular, to an active matrix display.
2. Background Art
In recent years, electroluminescence elements (hereinafter referred to as EL elements) have been drawing attention as light and thin surface light emitting elements. EL elements can be generally categorized into organic EL elements, which emit light when a direct current voltage is applied to a fluorescent material made of an organic material so as to recombine electrons and holes, and inorganic EL elements, which emit light from a fluorescent material made of an inorganic material in the process of returning to a relaxed state from excitation generated when an alternating current voltage is applied to the inorganic fluorescent material so that accelerated electrons in an electrical field as intense as approximately 106 V/cm collide against the center of light emission of the inorganic fluorescent material.
Furthermore, inorganic EL elements include dispersion type EL elements having a phosphor layer, where inorganic fluorescent material grains are dispersed in a binder made of a polymer organic material, and thin film type EL elements, where a dielectric layer is provided on one or both sides of a thin film phosphor layer having a thickness of approximately 1 μm. Dispersion type EL elements consume little power and are easy to manufacture, and thus have such an advantage that the cost of manufacture is low, and thus have been drawing attention. Conventional dispersion type EL elements have a multilayer structure and are formed of a substrate, a first electrode, a phosphor layer, a dielectric layer and a second electrode, which are layered in this order. The phosphor layer has a structure where inorganic fluorescent material grains, such as of ZnS:Mn, are dispersed in an organic binder, and the dielectric layer has a structure where a strong insulator, such as BaTiO3, is dispersed in an organic binder. An alternating current power source is installed between the first electrode and the second electrode, so that the dispersion type EL element emits light when a voltage is applied across the first electrode and the second electrode from the alternating current power source.
In the structure of dispersion type EL elements, the phosphor layer is a layer determining the brightness and efficiency of the dispersion type EL element, and inorganic fluorescent material grains having a grain size of 15 μm to 35 μm are used in the phosphor layer, as described in WO03/020848. In addition, the color of light emitted from the phosphor layer of dispersion type EL elements is determined by the inorganic fluorescent material grains used in the phosphor layer, and in a case where ZnS:Mn is used for the inorganic fluorescent material grains, for example, the emitted light is orange, while in a case where ZnS:Cu is used for the inorganic fluorescent material grains, the emitted light is bluish green. Thus, the color of emitted light is determined by the inorganic fluorescent material grains used, and therefore, in a case where light of another color, for example white light, is emitted, an organic pigment is mixed into the organic binder so as to convert the color of emitted light to another color, so that the emitted light has the target color, as described in Japanese Patent Laid-open Publication H7-216351.
However, there is a problem, such that the light emitted by light emitting bodies used in dispersion type EL elements has a low brightness, and the life of the light emitting bodies is short.
A method for increasing the voltage applied to the phosphor layer is possible as a means for increasing the brightness of emitted light. In this case, there is another problem, such that the half-life of light emitted from the phosphor material shortens in inverse proportion to the applied voltage. Meanwhile, a method for lowering the voltage applied to the phosphor layer is possible as a means for elongating the half-life, that is to say, elongating the life, but there is a problem, such that the brightness of emitted light lowers. Thus, the brightness of emitted light and the half-life are inversely related, so that when one is improved by increasing or decreasing the voltage applied to the phosphor layer, the other deteriorates. Accordingly, either the brightness of emitted light or the life (the half-life of light output) must be selected. Here, in the present specification, the half-life is the time it takes for the light output to reduce to half of the original output in the brightness of emitted light.
Therefore, it has been proposed that EL elements be made to emit light at a low voltage, as described in Japanese Patent No. 3741157. The EL element 50 shown in FIG. 33 is a method for light emission where a voltage is applied across electrodes 52 and 54 between which a phosphor layer 53 is inserted, the phosphor layer 53 dispersing phosphor material grains 61 made of microscopic CdSe crystal in a medium of indium tin oxide 63, which is a transparent conductor. This EL element 50 is a current injection type light emitting element, and thus characterized by being operable at a low voltage.